Left and right side sills of a vehicle body are positioned on the left and right sides of the vehicle body, and the side sills extend in a longitudinal direction. Requirements of the left and right side sills in all of the following three cases are that it be possible to guarantee a certain strength, and also that it be possible to guarantee collision force absorbing performance. The first is a case of a so-called front-surface collision, in which collision force acts on the front surface of the vehicle. The second is a case of a so-called rear-surface collision, in which a collision force acts on the rear surface of the vehicle. The third is a case of a so-called side collision, in which collision force acts on the side surface in the width direction of the vehicle.
For example, when the vehicle undergoes a side collision, it is possible that the collision force from the side could act on a center pillar extending upward from a longitudinally intermediate point of the side sill. The collision force being transferred from the center pillar to the side sill causes the side sill to twist into the passenger compartment. The center pillar collapses into the passenger compartment along with the twisting of the side sill. Such collapsing of the center pillar is preferably suppressed as much as possible.
A technique whereby the strength of the left and right side sills is increased is known from Japanese Patent Publication No. 3159231. In the technique known from Japanese Patent Publication No. 3159231, the left and right side sills are configured from hollow members having uniform cross sections, and the side sills have side sill reinforcing members inside the front half portions of the side sills. The cross sections of the side sills are closed cross sections having substantially square shapes. The strength of the side sill reinforcing members gradually decreases from the front of the side sills to the rear. Therefore, when the vehicle undergoes a front-surface collision, stress concentration in the side sills can be prevented.
When a common vehicle undergoes a front-surface collision, the front part of the vehicle body deforms to the rear, whereby the front wheels move rearward together with the front part of the vehicle body and come in contact with the front ends of the side sills. At this time, the front wheels are not limited to coming in contact with the entire surfaces of the side sill front ends.
As described above, in the technique known from Japanese Patent Publication No. 3159231, the front ends of the side sills are formed into closed cross sections having substantially square shapes. The square-shaped closed cross sections are composed of four sides: a vertical inner upright side positioned near the center of the vehicle width direction, a top lateral side extending outward in the vehicle width direction from the top end of the inner upright side, an outer upright side extending downward from the distal end of the top lateral side, and a bottom lateral side extending to the bottom end of the inner upright side from the bottom end of the outer upright side. A front wheel that has collapsed backward has a high possibility of coming in contact with only the inner upright side and the top lateral side. In other words, the front wheel has a high possibility of partially coming in contact with the front end of the side sill. In this case as well, there are requirements that the side sill have greater collision force absorbing performance and that the strength of the side sill be ensured. Moreover, weight reduction of the vehicle has also recently come to be a requirement.
In view of this, there is a demand for a technique which makes it possible to reduce the weight of the left and right side sills while ensuring the strength of the left and right side sills.